Spur tipped spade-type drill bits are commonly employed for drilling relatively large holes in wood and other similarly cutting resistant materials. These rather low cost alternatives to more costly, but more precise, conventional auger drill bits are easier to manufacture and are to some extent disposable. Hence, although relatively imprecise, spade drill bits are desirable in many situations.
FIG. 1 illustrates a typical steel spade bit 10 which includes an elongated shank or shaft 11 with one end formed for removable receipt in a drill assembly (not shown) during operational use. The opposite end includes a plate-like spade bit portion 12 forming a cutting edge 13 at the distal end thereof. The cutting edge 13 of more recent spade-bit designs include a centering tip 14 as well as a pair of spur tips 15, 15' on opposite sides of the centering tip. Typically, the downward angled edges 16, 16', forming centering tip 14, and the spur tip edges 17, 17', forming a portion of spur tips 15, 15', intersect one another at respective corner portions 18, 18' between centering tip 14 and the corresponding spur tip 15, 15'. During operation, the centering tip centers the drill bit during rotational cutting. Subsequently, the spaced-apart spur tips engage the material to be cut to define the perimeter of the bore before the portions between the center tip and the spur tip are excavated. Typical of these patented spur tipped spade-type drill bits may be found in U.S. Pat. Nos: 5,221,116; 5,061,127; 4,950,111; and 4,595,322.
While these spade-type drill bits adequately bore holes in woods and other relatively soft materials, problems occur when these relatively low operating speed bits are employed to cut harder and more abrasive materials which include particle board, MDF, and plywoods, and these same materials faced with plastic laminates such as urea resin or melamine resin based particle boards and plywoods. The increased abrasiveness of these materials to be cut, in combination with the low operating speed, causes these materials to easily dull the drill cutting edges of the bit.
Further, the discrete corner portions 18, 18' below spur tips 15, 15' and centering tip 14 cause significant problems during the drilling operation. As the chips are cut or sheared from a block of material, the cut chips compete for space in these corners where they become wedged therein. Hence, greater drilling pressures and forces in the axial direction are required to compensate for this space competition.
Moreover, the formation of the corner portions themselves increase the overall length or path of each combined edges (i.e., downward edge 16, 16' and spur tip edge 17, 17', respectively). This substantially increases the operating friction, thus generating heat, and reduces the cutting capability of the blade. Accordingly, to cut the above-mentioned more abrasive materials, this combination also necessitates a substantial increase in the operating pressures and forces in the axial direction, as well as larger power and torque requirements of the drilling assembly.
To drill hard composites or more abrasive materials such as urea resin or melamine resin based particle boards and plywoods, solid carbide or carbide tipped brad-point drills (FIG. 2) are often employed. These fluted drill bits satisfactorily bore holes in these materials but require higher operating speeds of the drill, are more costly, and are difficult to resharpen.